Nanoparticulate silica colloids have been known for many years and have many uses in industrial and consumer products. A nanoparticle has dimensions on the order of a molecular scale, typically on the order of about 1 to 100 nm, or 10−9 to 10−7 meters. For present purposes, colloidal particles having an average diameter of about 300 nm or less are relevant.
Colloidal particles find use in a broad variety of applications such as pigments for paints, in cosmetic products, in paper products, as polishing media in semiconductor electronics, to name only a few. An application of particular importance is the use of colloids in the production of inkjet media.
In a typical inkjet recording or printing system, ink droplets are ejected from a nozzle at high speed towards a recording element or medium to produce an image on the medium. The ink droplets, or recording liquid, generally comprise a recording agent, such as a dye or pigment, and a large amount of solvent. The solvent, or carrier liquid, typically is made up of an aqueous mixture, for example, comprising water and one or more organic materials such as a monohydric alcohol, a polyhydric alcohol, or the like.
An inkjet recording element typically comprises a support having on at least one surface thereof at least one ink-receiving layer. There are generally two types of ink-receiving layers (IRL's). The first type of IRL comprises a non-porous coating of a polymer with a high capacity for swelling, which non-porous coating absorbs ink by molecular diffusion. Cationic or anionic substances may be added to the coating to serve as a dye fixing agent or mordant for a cationic or anionic dye. Typically, the support is a smooth resin-coated paper and the coating is optically transparent and very smooth, leading to a very high gloss “photo-grade” inkjet recording element. However, this type of IRL usually tends to absorb the ink slowly and, consequently, the imaged receiver or print is not instantaneously dry to the touch.
The second type of ink-receiving layer or IRL comprises a porous coating of inorganic, polymeric, or organic-inorganic composite particles, a polymeric binder, and optional additives such as dye-fixing agents or mordants. These particles can vary in chemical composition, size, shape, and intra-particle porosity. In this case, the printing liquid is absorbed into the open interconnected pores of the IRL, substantially by capillary action, to obtain a print that is instantaneously dry to the touch. Typically the total interconnected inter-particle pore volume of porous media, which may include one or more layers, is more than sufficient to hold all the applied ink forming the image.
Basically, organic and/or inorganic particles in a porous layer form pores by the spacing between the particles. The binder is used to hold the particles together. However, to maintain a high pore volume, it is desirable that the amount of binder is limited.
A porous inkjet recording medium that is glossy usually contains at least two coated layers in addition to the support: a base layer nearer to the support and a glossy image-receiving layer further from the support. Layers comprising smaller particles, for example inorganic particles, as described in U.S. Pat. No. 6,630,212 to Bermel et al. or U.S. Pat. No. 6,641,875 to Sadasivan et al., are capable of producing high gloss levels. Colloidal particles, as used herein, can refer to either (1) colloidal secondary particles or aggregates of primary particles, such as in the case of fumed metallic oxide, for example, fumed alumina or fumed silica, or (2) colloidal primary particles, for example, hydrated alumina or colloidal silica.
U.S. Pat. No. 5,116,535 to Cochrane discloses a method of preparing an aqueous dispersion of fumed silica at concentrations of at least about 35 weight % by high shear mixing followed by slight dilution to the desired concentration, such that the dispersion in the mixer, before dilution, will have a fumed silica concentration at least about 5% greater than the desired final concentration of fumed silica in the aqueous colloidal dispersion of fumed silica. The aqueous dispersion may be passed through a filter to remove grit and any agglomerated particles. Low surface area fumed silica (less than 75 m2/g) is preferred. An aqueous colloidal dispersion having a viscosity of below about 1000 centipoise that will not gel for at least 2 hours is obtained without a stabilizer such as an alkali or base.
U.S. Pat. No. 5,246,624 to Miller et al. discloses a similar method additionally including pre-acidifying the aqueous medium and, after dispersing the fumed silica, adding a pH-increasing stabilizer such as ammonium hydroxide. Again, low surface area fumed silica (less than 75 m2/g) is preferred.
U.S. Pat. No. 4,599,363 to Miles et al. discloses a method for making a paste comprising a vehicle of humectant and water, with water content below 35% and a bodying agent such as colloidal or fumed silica (referred to as “pyrogenic silica”) at a concentration below 20%. The solids are introduced into the closed mixing vessel, by vacuum induction, to the area of highest turbulence at the bottom of the mixing tank. The mixer disclosed is a mixing screw appropriate for a paste, distinct from a high-shear mixer for preparing a coating dispersion of fumed silica as in previously mentioned prior art patents. In the paste of Miles et al., the principal particles are abrasives comprising silica gels with particle sizes significantly above 1 micron, which particles are unsuitable for glossy photo-quality inkjet media. Also, the high-humectant level composition is unsuitable for forming a porous ink-receiving layer.
U.S. Pat. No. 6,695,907 to Scharfe et al. discloses a method of preparing a dispersion of fumed silica that has been doped with AlCl3 using a rotor-stator system. In reference to Table 1 of U.S. Pat. No. 6,695,907, the examples of low surface area silica were dispersible, but it was not possible to produce a 40% solids dispersion of AEROSIL 90 silica (90 m2/g as measured by BET) using this system.
U.S. Pat. No. 6,403,162 to Tokunaga et al. discloses a method for making an inkjet recording material comprising dispersing fumed silica. A cationic polymer having a molecular weight of 10,000 or less is added to an aqueous medium prior to the introduction of the fumed silica. The cationic polymer is used to prevent agglomeration of fine silica particles added as a solid powder. U.S. Pat. No. 6,403,162 discloses primary mixing to prepare a slurry using propeller stirring, turbine type stirring, homomixer type stirring, etc., and then secondary mixing carried out with a high-pressure homogenizer, a ball mill, or the like. The use of high-pressure homogenizer treatment for a fumed silica of surface area 300 m2/g in a comparative example without cationic polymer resulted in aggregation and sedimentation after six days.
European Patent No. 1,228,891 to Ohya et al. discloses, for use in making an inkjet recording element, the preparation of a low-solids dispersion of AEROSIL 300 silica by vacuum induction using, for example, a JET STREAM Inductor Mixer TDS manufactured by Mitamura Riken Kogyo Co. However, a disadvantage of using a low-solids dispersion in a coating is that drying may require harsh conditions which may result in loss of product quality or may necessitate a long, expensive drying process.
U.S. Pat. No. 6,676,719 to Lortz et al. discloses, for use in various applications, including coating of paper and chemical-mechanical polishing of semiconductor substrates, a process of pre-dispersing and wetting a fumed silica, employing a high pressure homogenizer, for example, dispersion and suction mixer from Ystral. Dispersion was completed with a Z 66 type rotor/stator from Ystral. Lortz et al. found that alkali doped silica provided greater stability than undoped silica. In a comparative example using AEROSIL 130 silica, it was not possible to draw in the entire quantity of solids above 20 weight % due to high viscosity caused by partial agglomeration. Potassium doping was proposed as a solution to substantially improve the wetting or pre-dispersion time.
Japanese patent publication No 2006-076841 by Takaaki et al. describes a two-step method for dispersing wet-process silica powder comprising, as a first or primary dispersion process step in order to obtain a silica slurry, a flow-type vacuum dispersing agitator continuously or intermittently supplying and dispersing the silica to a mixture of a dispersing agent and an aqueous dispersing medium and, as a second dispersion process step, pulverizing the silica slurry.
There is a need for an improved low-cost, efficient, industrial-scale method of preparing colloidally stable, nanoparticle dispersions of fumed silicas. Efficient wetting and dispersing of low-bulk-density, high-surface-area powders for use in high-solids coatings for glossy inkjet photo-quality media, and prevention of particle agglomeration, are particular concerns.